(a) Field of the Invention
This invention relates generally to a circuit for driving an AC plasma display panel, and more particularly, to a charge-limiting energy recovery driving circuit with an improved structure in which the current during the sustain period is limited to obtain higher power efficiency.
(b) Description of the Related Art
A panel structure of a conventional AC PDP is shown in FIG. 1a. As shown in the figure, the panel consists of a front glass plate 1 and a rear glass plate 2. On the rear glass plate 2, a plurality of address electrodes A are parallelly disposed. A thin dielectric layer 5 is then placed on top of these address electrodes A, covering the entire surface of the rear glass plate 2. On top of this dielectric layer 5, barrier ribs 3 are formed, creating “cells.” In addition, a layer of light-emitting phosphor 4 (red, blue, or green) is applied onto the sidewalls of the barrier ribs 3 and the dielectric layer 5.
On the front glass plate 1, a plurality of scan electrodes Y and a plurality of sustain electrodes X are formed. The scan electrodes and the sustain electrodes are parallelly aligned, but perpendicular to the aforementioned address electrodes A of the rear glass plate 2, forming a grid. Much like the rear glass plate 2, a dielectric layer 8 is then placed on top of the electrodes X and Y, covering the entire surface of the front glass plate 1. A thin MgO layer 9 is then placed on top of this dielectric layer 8.
An AC PDP is then created by combining these two glass plates 1 and 2 together, evacuating the space between the two plates 1 and 2, and filling this gap with a mixture of rare gases (typically Xe—Ne).
As in FIG. 1b, each of the discharge cells is formed at each of the intersections of each of the scan electrodes and each of the sustain electrodes of the front glass plate 1 and each of the address electrodes of the rear glass plate 2. Because the aforementioned “cells” on the rear glass plate are coated with red, blue, or green phosphor, full-color images can be obtained.
In the case of an AC PDP (as in FIG. 2), a subfield method is typically employed to realize gradation luminance representations and thus to achieve 256 gray scale.
The subfield method divides one field period into N subfields, each of which is allocated a light emission period (the number of times of light emission) corresponding to a weighting for each bit digit of pixel data (composed of N bits) to drive the PDP to emit light. Each of the subfields consists of a reset period, an address period, and a discharge-sustaining period (a sustain period). In other words, each of the eight subfields mentioned above has one length of the sustain periods of 20, 21, 22, 23, 24, 25, 26 and 27, respectively. These subfields are combined to realize 256 gray-scale in a TV field of an AC PDP.
The reset period makes the wall charge accumulated on each of the whole cells substantially equal. In order to accomplish this, the reset period erases previous image information, thereby making the initial condition of the whole cells equal, before writing new image information. The address period following the reset period has a function of selecting each of the cells whether to be turned on or off. It accomplishes this by forming a positive wall charge on the scan electrode and a negative wall charge on the address electrode of the selected cells to be turned on. The wall charges are formed by a discharge between the scan electrode Y and the address electrode A intersecting at each cell. During the sustain period following the address period, a sustain voltage lower than the firing voltage is applied between the scan electrode Y and sustain electrode X. Thus, sustain discharges can be fired and maintained only in those cells turned on during the address period. In other words, only those cells with a wall charge accumulates on the sustain electrode can continue the sustain discharge after the aforementioned address period.
For driving the AC PDP during the sustain period, high voltage pulses are applied alternately to the panel capacitance. Generally, in the driving circuit of an AC PDP, the energy recovery driving circuit is employed. This circuit increases the efficiency of the system by recovering the energy in the panel capacitance. In addition, it helps to reduce noise due to an electromagnetic interference (EMI), and makes the driving of the sustain period stable and efficient.
There have been numerous efforts to improve the energy efficiency of the AC PDP, but the low energy efficiency is still a major problem. A very low percentage of the total electric power consumed in the AC PDP is actually converted into useful visible photons. The vast majority of energy is consumed during the sustain period. The majority of the current flowing into an AC PDP is the current flowing through the panel capacitance at the time of discharge firing.